The present invention relates to a method for removing gaseous impurities from feed gas streams in a temperature swing adsorption pre-purification unit. More particularly, the present invention provides for a method to remove H2, H2O, CO, CO2 and additionally hydrocarbons and oxides of nitrogen from gas feedstreams such as air.
High purity gases such as nitrogen in which impurities are present in amounts well below part per million levels are required in the manufacture of integrated circuits to prevent defects in chips of increasing line densities. Cryogenic distillation is typically used for the production of highly purified nitrogen gas.
Removal of impurities from the feed gas for cryogenic distillation is required for the production of high purity nitrogen. When air is used as the feed gas, impurities, such as H2O and CO2, have to be removed to prevent freeze-up in the low temperature sections of the plant while other impurities, such as H2 and CO, have to be removed to prevent contamination of the nitrogen product.
A two-step procedure has been employed for the removal of these impurities from air in a nitrogen production process. In the first step, a compressed feed gas is heated to temperatures between 150xc2x0 to 250xc2x0 C. and then contacted with a catalyst to oxidize CO to CO2 and H2 to H2O. Noble metal catalysts, typically based on platinum, are commonly used for the oxidation step. In the second step, the oxidization products, CO2 and H2O, are removed from the compressed gas stream either by a temperature-swing or pressure swing adsorption process.
These processes, although effective, are disadvantageous for the commercial scale production of highly purified gases, particularly nitrogen gas due to their high cost of operation. The cost of operation is high because of the extensive use of expensive noble metal catalysts. In addition, separate vessels must be used for the catalytic treatment step and the adsorption step to remove the impurities. In addition, heat exchangers are required to both heat the gas as it passes into the catalyst vessel and cool the effluent therefrom. This poses additional costs, both in terms of equipment and energy.
Low temperature processes for removing parts per million levels of impurities from inert gas streams are also known in the art.
The present inventors have discovered that the use of a multi-layer bed where the trace impurities, primarily CO2, are removed prior to contacting the gaseous feedstream with the H2 catalyst will improve the performance of the H2 catalyst. This not only improves overall performance but also reduces the amount of H2 catalyst necessary in the process.
The present invention provides for a process for producing a gaseous product substantially purified from impurities comprising the steps of:
a) removing water vapor from a gaseous feedstream containing impurities;
b) contacting the gaseous feedstream with an oxidation catalyst to convert CO to CO2;
c) removing CO2 from the gaseous feedstream of steps (a) and (b);
d) contacting the gaseous feedstream of step (c) with an oxidation catalyst to convert H2 to H2O; and
e) removing H2O from the gaseous feedstream of step (d), thereby obtaining the substantially purified gaseous product.
Optionally, an additional stage can be added between steps (c) and (d) whereby hydrocarbons and oxides of nitrogen are removed from the gaseous feedstream.
In another embodiment of the present invention, a gaseous product substantially purified from impurities is obtained by:
a) removing water vapor from the gaseous feedstream containing impurities;
b) removing CO2 from the gaseous feedstream of step (a);
c) contacting the gaseous feedstream of step (b) with an oxidation catalyst to convert CO to CO2;
d) contacting the gaseous feedstream of step (c) with an oxidation catalyst to convert H2 to H2O;
e) removing H2O and CO2 from the gaseous feedstream of steps (c) and
(d), thereby obtaining the substantially purified gaseous product.
Optionally, the additional stage to remove hydrocarbons and oxides of nitrogen from the gaseous feedstream may be added between steps (b) and (c).
Typically, the gaseous product obtained is air in a temperature swing adsorption (TSA) process prior to the air being fed to a cryogenic distillation unit in an air separation unit (ASU). The regeneration gas employed in a typical TSA process needs to be free of H2, CO, CO2, H2O, oxides of nitrogen and hydrocarbons.
The gaseous feedstream is treated in a single treatment zone, preferably in a single vessel which includes two catalyst sections and three adsorbent sections. The first section contains one or more beds of a water-removing adsorbent such as activated alumina, silica gel, zeolites, or combinations thereof. The first catalytic layer for converting CO to CO2 may be a mixture of manganese and copper oxides or nickel oxides.
The second adsorbent section contains an adsorbent for removing CO2 from a gaseous feedstream. This adsorbent may be zeolites, activated alumina, silica gel and combinations thereof.
The section of the vessel used for converting H2 to H2O contains an oxidation catalyst. This catalyst is preferably a noble metal catalyst such as supported palladium. The last layer contains an adsorbent for removing water and/or carbon dioxide which may be zeolites, activated alumina, silica gel and combinations thereof.